To reset: push the button on the left stalk until you get to the mpg display, then push it and hold it in. It does an average for the whole tank (or from the last time you did the above and reset the calculation), so it won't change instantanteously.

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2005 X3 3.0i, manual, mystic blue, matte titanium interior trim, gray leather (premium pkg), heated seats, chrome exhaust tips!